Flat panel display devices, such as an organic light emitting display (OLED), a plasma display panel (PDP), or a liquid crystal display (LCD), have been actively developed and are increasingly preferred over the heavy and bulky displays based on cathode ray tube (CRT) technology. In general, a PDP displays characters and/or images by using plasma obtained by a gas discharge, while an OLED displays characters and/or images by generating electronic luminescence of specific organic and/or polymeric materials. Furthermore, in general, an LCD displays characters and/or images by changing an intensity of light passed through the liquid crystal layer, which is located in between two conductive layers and controlled by the current therebetween.
Among the flat panel displays, both LCD and OLED have a display portion and a gate driving portion. The display portion includes pixels, having a switching element in each pixel, and signal wires for transmitting signals to the switching element of the pixels. The gate driving portion includes shift registers for switching on and off the pixel elements.
Each of the shift registers includes a plurality of interconnected stages. Each stage may include an input portion, an output portion, and a discharging portion for transmitting signals according to the combination of the output signal of the previous or latter stages and a clock signal.
The output portion of the stage may include a number of transistors, which are connected to the gate wires and to the other stages. For these transistors, a gate signal output transistor consumes approximately 40 to 50 percent of the whole stage area to have sufficient power to transmit signals onto the gate lines and to former stages. Furthermore, for example, to minimize ripple, the gate signal output transistor (e.g., NMOS transistor) generally has more capacitance between its gate and drain than between its gate and source.
Accordingly, to provide sufficient capacitance between the gate and the source, the gate and the source are enlarged to provide a broader overlap; thus, the gate signal output transistor's area is automatically increased based on the capacitance required. For this reason, the design rules may not permit adequate margin for circuit design and architecture, depending upon the design requirements or desired design application. Also, the broader overlap raises the possibility of a short circuit between the source and gate of the gate signal output transistor that will result in a gate signal output failure.
This drawback may be especially apparent when an application has a narrow black matrix area, resulting in the display device (e.g., for a notebook computer) not having enough area for the manufacturing seal process (i.e., the adhesion process of the upper plate and the lower plate of the display device). Accordingly, there is a need for a display device with a gate driving portion that may overcome one or more of the disadvantages of conventional systems, such as those discussed above.